The present invention relates to an operational amplifier arrangement which is adapted to control the input offset voltage of an operational amplifier included therein as is described in the non-characterising portion of the first claim.
Such an operational amplifier arrangement is already known in the art, e.g. from U.S. Pat. No. 6,066,986. Therein an integrated operational amplifier device is described which includes an operational amplifier, and an input offset control circuit including means connected to this operational amplifier for providing at least one of a positive and a negative electrically adjustable input offset voltage devoid of ac. An external programming device is coupled to a user input adjust terminal, and further devices comprised within the input offset control circuit and coupled between this external programming device and the operational amplifier ensure a voltage applied to a specific bias circuit of the amplifier such that the desired output voltage is obtained.
It is however not mentioned how the adjustable input offset voltage which is the input to the external programming device, is determined. The relationship between a sensed output voltage, and the input signal to the external programming device for further delivery to the bias circuit of the amplifier is not mentioned.
Known offset control circuits thereby rely on measurements of the output voltage of the operational amplifier, whereby a comparison with a known reference voltage is performed, such as for instance described in the tutorial handbook on Analog Circuit Design such as e.g. xe2x80x9cDesign of Analog CMOS Integrated Circuitsxe2x80x9d by B. Razavi, pp 471-476.
However these existing solutions are not suited to handle the accuracy required by e.g. state-of-the-art xDSL line drivers. These drivers may see a very low resistive load, for instance merely 5-6 Ohms. For these applications a small output offset voltage results rapidly in large offset load currents, thereby increasing unnecessarily the power consumption and the distortion level of these output drivers.
An object of the present invention is therefore to provide an operational amplifier arrangement of the above known kind, but whereby a very high level of accuracy is obtained, allowing these operational amplifiers to be used for driving small loads.
According to the invention, this object is achieved due to the fact that two individual output currents in two series branches of the output stage of the amplifier are compared with each other, as is further described by the characterizing portion of the first claim.
In this way, by not just comparing the output currents with a predetermined reference, but with each other, and by further adjusting the input bias voltages of the operational amplifier such that these measured output currents become equal, a much higher accuracy is obtained.
Other characteristic features of the present invention are described in other claims.
The input offset control circuit thereby consists of a control logic circuit and a D/A converter. Compared to the prior art where a capacitor is used for storing a charge for further tuning the offset voltage, this solution does not use any capacitor. The charge stored on capacitors indeed needs to be refreshed because of leakage which presents an inherent drawback. This problem is thus also overcome with the present invention.
The above and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawings wherein: